Cholangiocytes are the target cells in chronic cholestatic liver diseases such as primary biliary cirrhosis (PBC), and primary sclerosing cholangitis (PSC), which are characterized by the damage/ proliferation of cholangiocytes of different sizes and function. Cholangiocyte proliferation and regrowth are critical for the maintenance of biliary mass and the functional recovery during the pathogenesis of these cholangiopathies. The elucidation of the intracellular mechanisms regulating the proliferative/regenerative responses of large and small bile ducts to cholestasis will play a pivotal role in the development of therapeutic strategies for the treatment of cholestatic liver diseases. Secretin (SEC), a gastrointestinal hormone, is critical for hepatobiliary growth and regeneration. As the secretin receptor (SR) is only expressed by cholangiocytes in the liver, levels of cyclic-AMP in the bile ducts increase and bicarbonate rich bile is secreted after administration of secretin. During hepatobiliary growth/damage, cholangiocytes acquire neuroendocrine phenotypes and thereby secrete and respond to neuroendocrine hormones such as VEGF/NGF and secretin that regulate biliary mass and response to damage. Targeting specific factors that are activated/deactivated during liver injury may help limit biliary damage and the progression of PBC, PSC and liver fibrosis. We have shown that SEC/SR (expressed only by cholangiocytes) axis regulate biliary mass during bile duct ligation (BDL). Secretin levels are elevated during the early stages of BDL. Lack of SR expression leads to: (i) ablation of biliary growth during BDL; and (ii) exacerbation of biliary damage in response to toxins indicating that SEC is a pro-proliferative/protective factor. The mechanisms regulated by the SEC/SR axis during models of acute (short-term BDL and CCl4 administration) and chronic biliary damage (animal models of PSC and PBC) are unknown. Preliminary data indicate that: (i) the SEC/SR axis has pro-proliferative and protective functions during biliary injury (e.g., CCl4 administration and in dnTGFbetaRII KO mice); (ii) activation of the SEC/SR axis (which increases biliary growth) results in the downregulation of the miRNAs, let-7a and miR125-b, that modulate the expression of VEGF and nerve growth factor (NGF) as well as the other potential target genes such as Bcl-2; and (iii) the serum levels of SEC are reduced in PBC and PSC patients indicating that dysregulation of the SEC/SR axis may be key in the pathogenesis of these cholangiopathies. We propose the overall hypothesis that the SEC/SR axis is a key pathway responsible for mediating biliary proliferation/damage during liver diseases. Our long-term objective is to define the therapeutic role of the SEC/SR axis in hepatobiliary disorders such as PBC/PSC. The overall hypothesis will be evaluated by three specific aims. First, we will demonstrate that the SEC/SR axis modulates cholangiocyte proliferation and damage during animal models of acute cholestasis through the activation of autocrine/paracrine mechanisms. Second, we will identify functional SEC-dependent miRNAs involved in the regulation of the expression levels of the pro-proliferative/protective neuroendocrine factors. Third, we will determine the trophic/protective SEC/SR axis that contributes to biliary damage/recovery during chronic cholestasis in animal models of PSC and PBC. Therapeutic effects of secretin dependent gene/miRNA manipulation on biliary cell growth and proliferation will be evaluated. Novel insights into the physiological roles and mechanisms of molecular and functional secretin signaling in human biliary epithelium will be obtained. Meanwhile, the fundamental knowledge obtained in the regulation of growth, proliferation and apoptosis in biliary tree is expected to advance the field of biliary biology/ pathophysiology. !